The present invention relates to a braking pressure control unit for vehicular hydraulic brake systems in which the brakes at the rear wheels of the vehicle are pressurized by two separate brake circuits. The control unit is formed by two pressure control valves coaxially arranged in series and each controlling the pressure medium connection between the pressure medium source and the wheel brake cylinders of one brake circuit, one pressure control valve being designed as a braking force reducer provided with a stepped piston and the other pressure control valve being designed as a braking force limiter.
Such a braking pressure control unit is described in German Pat. No. 2,236,294. In the braking pressure control unit described in this patent, both the closing member of the braking force reducer and the closing member of the opposite braking force limiter are supported by an intermediary piston. Upon pressurization of the braking force reducer, the stepped piston starts moving against its control force and will at first continue to do so until the closing distance of the closing member of the braking force reducer has been travelled. As soon as the braking force reducer closing member is closed, the intermediary piston will move towards the braking force reducer upon a further pressure increase since, on the side of the braking force limiter, the intermediary piston is acted upon by increasing pressure, while, on the side of the braking force reducer, the intermediary piston is acted upon by constant pressure. A state of balance is achieved in that the intermediary piston is permanently pushing the closing member of the braking force reducer back into its open position. In doing so, the intermediary piston is following the movement of the stepped piston. The intermediary piston will cease to follow the further displacement of the stepped piston of the braking force reducer only after the braking force reducer has travelled the closing distance of the closing member of the braking force limiter in addition to the closing distance of its own closing member, both closing members then being closed. Now, the phase of pressure reduction will be starting in both brake circuits.
Upon failure of either brake circuit caused by a defect, the displacement of the stepped piston by the amount of the closing distance of but one closing member will be sufficient to cause the closing member of the intact brake circuit to close.
Due to the reduction of the displacement distance by one half, the control force will correspondingly be smaller than in the case of the fully available displacement distance as a result of which the braking force will be reduced considerably upon the closure of the closing member of the intact brake circuit. This, however, is highly undesirable. If one brake circuit fails, it is desired to get at least the maximum available braking force out of the remaining brake circuit.